1. Field of the Invention
This invention relates to liquid crystal light valves (LCLVs), and more particularly to a high resolution LCLV and a method for fabricating the same.
2. Description of the Related Art
LCLVs operate by converting an input image to an output image in which the wavelength, intensity and/or coherence of the original input image are varied. Applications for this type of device include image amplifiers, optical data processing, wavelength converters and incoherent-to-coherent image converters. The basic principles of an alternating current LCLV are described in U.S. Pat. No. 3,824,002 issued to Terry D. Beard, "Alternating Current Liquid Crystal Light Valve". Overviews of silicon-based LCLVs are provided in Efron et al., "The Silicon Liquid-Crystal Light Valve", Journal of Applied Physics, Vol. 57, No. 4, Feb. 15, 1985, pages 1356-1368, and Efron et al., "The Applications of Silicon Liquid Crystal Light Valves to Optical Data Processing", Proceedings of SPIE - The International Society for Optical Engineering, Vol. 388, Jan. 20-21, 1983, pages 152-161.
A thin, single crystal silicon wafer, generally about 125 microns thick, has previously been used as a substrate in silicon-LCLVs. It is important that the surface of the wafer be flat for good optical quality, and further that a silicon electrode employed in the device by very thin (on the order of a few microns) to achieve high resolution. Unfortunately, the thin semiconductor wafers which it has been necessary to use have only a limited polishing capability, which results in a relatively poor surface quality. Wafer distortion due to high temperature processing, and a limited correction capability while mounting the wafer add to the problems of surface quality. The non-uniformity of the wafer surface is transferred to a liquid crystal layer which is mounted against the wafer, and limits the use of the device for optical processing and adaptive optics applications.
Departing from the field of LCLVs, a technique has been developed in the field of microelectronics in which a silicon film is epitaxially grown on a sapphire base. Called silicon-on-sapphire (SOS), in this technique the insulating sapphire base results in the effective elimination of cross-talk between circuit elements formed from the overlying semiconductor. Although there are differences in the lattice structures of silicon and sapphire, it has been discovered that a silicon film can be epitaxially grown from the sapphire base. In addition to the isolation provided between circuit elements, this approach also allows for high temperature processing. An improvement on the process which provides a higher quality epitaxial growth by significantly reducing the concentration of lattice defects near the sapphire substrate is disclosed in U.S. Pat. No. 4,509,990 to Vasudev, and assigned to Hughes Aircraft Company, the assignee of this invention. In the Vasudev process an amorphous buried layer is formed in the semiconductor near the insulator substrate by implanting an ion species into the semiconductor. The amorphous buried layer is then re-grown "from the top down" using the unamorphized portion further away from the sapphire substrate as a recrystallization seed. This provides a substantially uniform, high quality crystal structure throughout the semiconductor layer.
In addition to microelectronic circuits, the SOS technique is suggested for a liquid crystal photodiode in U.S. Pat. No. 4,198,647 to Grinberg et al., issued Apr. 15, 1980 and assigned to Hughes Aircraft Company. In this patent a junction is formed between a thin layer of p-doped silicon and a much thicker layer of n-doped silicon. The n-doped layer is described as being 5 mils thick, and there is no teaching of how the n-type layer is formed over the thin p-type layer. A suggestion is made for the use of a sapphire substrate, without elaboration as to how it might be implemented, in column 2, lines 54-61 as follows: "Although in one embodiment of this invention a silicon wafer film is used for the semiconductor body, other semiconductor material can also be used for the transfer medium. In addition, the silicon (or other semiconductor) body can be fabricated on an insulating substrate such as sapphire. For example, large area silicon diodes can be built epitaxially on a sapphire layer." Furthermore, the SOS technology at that time was not capable of yielding high resistivity, high uniformity silicon epitaxial layers with the thickness required for Si-LCLV applications.
Despite this activity, no way has yet been found of implementing a LCLV with the desired degree of flatness and high quality thin film silicon.